In recent years, various image forming apparatuses are suggested for forming a visual image on a recording medium, such as paper, in accordance with image signals. For example, Japanese Laid-Open Patent Application No. 6-155798/1994 (Tokukaihei 6-155798) discloses an image forming apparatus for directly forming a toner image on a recording medium with toner as image visualizing particles that is projected so as to travel through a space and adhere to the recording medium. Referring to FIG. 12, the following description will explain such a conventional image forming apparatus.
The conventional image forming apparatus, as shown in FIG. 12, includes an image forming section 101 composed of a toner supply section 102 and a print section 103. The image forming apparatus projects toner 201 at a sheet of paper 105 as a recording medium in a controlled manner in accordance with image signals, so that the toner 201 adheres to the sheet of paper 105. An image is directly formed on the sheet of paper 105 in this manner.
The toner supply section 102 is composed of a toner storage box 200 for storing the negatively charged toner 201 and a toner holding body 202 for holding the toner 201. The toner holding body 202 is grounded and rotates in the direction indicated by the arrow E in FIG. 12 at a rotation speed of 30 mm/sec, measured on the surface of the toner holding body 202. The toner 201 is a single component toner, and the particle diameter is 10 .mu.m in average. The toner 201 is charged to a charge value of -8 .mu.C/g to -10 C/g with a well-known technique. The toner 201 is held on the toner holding body 202 so as to form a layer of 80 .mu.m in average thickness on the outer peripheral surface thereof.
The print section 103 of the image forming apparatus 101 includes an opposite electrode 205 made of an aluminum pipe of 50 mm in diameter and a control electrode 206 disposed between the opposite electrode 205 and the toner holding body 202. The opposite electrode 205 is provided to be separated from the outer peripheral surface of the toner holding body 202 by a distance of 1 mm. The opposite electrode 205, supplied with a high voltage of about 2 kV by a D.C. power source section 210, rotates in the direction indicated by the arrow F in FIG. 12 at a rotation speed of 30 mm/sec, measured on the surface of the opposite electrode 205. In other words, an electric field necessary for projecting the toner 201 held on the toner holding body 202 towards the opposite electrode 205 is formed between the opposite electrode 205 and the toner holding body 202.
The control electrode 206 extends in two dimensions and opposes parallel to a tangential plane on the surface of the opposite electrode 205. The control electrode 206 has a structure through which the projected toner 201 can pass and travel from the toner holding body 202 towards the opposite electrode 205. The projection of the toner 201 from the toner holding body 202 towards the opposite electrode 205 is controlled by changing the potential applied to the control electrode 206 that, in turn, changes the electric field generated between the toner holding body 202 and the opposite electrode 205.
The control electrode 206 is disposed to be separated from the outer peripheral surface of the toner holding body 202 by a distance of 100 .mu.m. The control electrode 206 is composed of a 50 .mu.m thick flexible print circuit (FPC) 206a and ring-shaped electrodes 207 made of a 20 .mu.m thick copper foils. The substrate 206a is provided with gates 209 of 150 .mu.m in diameter through which the toner 201 passes. The ring-shaped electrode 207 is provided so as to encircle the gate 209. Each of the ring-shaped electrodes 207 is electrically connected to a control power source section 211 via a power supply wire (not shown) and a high voltage driver (not shown).
A voltage is applied to the ring-shaped electrodes 207 by the control power source section 211 in accordance with an image signal. That is, when the toner 201 held on the toner holding body 202 is to pass towards the opposite electrode 205, the control power source section 211 applies a voltage of 200 V to the ring-shaped electrodes 207 for 300 .mu.sec. Meanwhile, the toner 201 moves from the toner holding body 202 to the opposite electrode 205 in 250 .mu.sec. Therefore, the voltage is applied for a long enough time for the toner 201 to travel from the toner holding body 202 to the opposite electrode 205.
On the other hand, when the toner 201 held on the toner holding body 202 is not to pass towards the opposite electrode 205, the control power source section 211 applies a voltage of -200 V to the ring-shaped electrodes 207. A potential applied to the control electrode 206 is controlled in accordance with an image signal in this manner. Then as the sheet of paper 105 is placed on the side of the opposite electrode 205 opposing the toner holding body 202, a toner image is directly formed on the surface of the sheet of paper 105 in accordance with the image signal.
A single trigger circuit (not shown) almost simultaneously applies a voltage for rotating the toner holding body 202 and the opposite electrode 205, a voltage to the control electrode 206 so as to prevent the toner 201 from passing, and a high voltage to the opposite electrode 205.
Incidentally, in the image forming apparatus described so far, the toner 201 is pulled away from the toner holding body 202 with a force generated by an electric field and projected towards the opposite electrode 205. Since forces such as an image force, a van der Waals force and a liquid bridge force are applied to the toner 201 and the surface of the toner holding body 202, the toner 201 adheres to the toner holding body 202 with adhesive forces of certain strength. Therefore, in the image forming apparatus described so far, a strong electric field is needed for projecting the toner 201 against the adhesive forces.
For this purpose, for example, Japanese Laid-Open Patent Application No. 4-358856/1992 (Tokukaihei 4-358856) discloses a configuration which weakens the adhesive forces of the toner and thereby needs a weaker electric field for projection of the toner. A toner holding body 302 disclosed in this laid-open patent application has a structure in which, as shown in FIG. 13, a piezoelectric member 304 coats the surface of a metal roller core 303. The metal roller core 303 is connected to a vibration power source 307. The vibration power source 307 can apply a vibration voltage to the metal roller core 303 and a reference electrode layer 306 disposed on the side of an aperture electrode 305 facing the toner holding body 302, thereby generating a vibration electric field therebetween.
With the configuration, as a toner supply roller 308 rotates, the toner 301 in a toner box 300 adheres to and is held on the toner holding body 302. Thereafter, the toner holding body 302 rotates and transports the toner 301 to a neighborhood of an aperture 305a of the aperture electrode 305. Here, the piezoelectric member 304 provided between the metal roller core 303 and the reference electrode layer 306 elongates and shortens with application of the vibration voltage by the vibration power source 307. As a result, the surface of the toner holding body 302 vibrates, stirring the toner 301 adhering to the toner holding body 302 and considerably weakening the adhesive forces of the toner 301 to the toner holding body 302. In this manner, the laid-open patent application weakens the adhesive forces of the toner 301 and thereby reducing the voltage necessary for projecting the toner 301.
Meanwhile, another example is disclosed by Japanese Laid-Open Patent Application No. 4-168064/1992 (Tokukaihei 4-168064), in which a vibration plate 403 is connected to a piezoelectric vibrator 402 and disposed on the back side of a toner transport belt 401 (a belt-shaped toner holding body) as shown in FIG. 14. A cam roller (not shown) may be used instead of the piezoelectric vibrator 402. In this laid-open patent application, vibration of the piezoelectric vibrator 402 (or rotation of the cam roller) vibrates the vibration plate 403, which in turn vibrates the toner transport belt 401. As a result, the adhesive forces of the toner (not shown) adhering onto the toner transport belt 401 are weakened.
Nevertheless, with the configurations disclosed by Japanese Laid-Open Patent Applications No. 4-358856/1992 and No. 4-168064/1992, the piezoelectric member 304, the piezoelectric vibrator 402, the cam roller, etc. are used to vibrate the toner holding body 302 and the surface of the toner transport belt 401. Therefore, such conventional configurations need to employ a special arrangement, such as a drive circuit and mechanism, a power supply, and a vibration electric field, only for the purpose of driving the piezoelectric member 304, the piezoelectric vibrator 402, the cam roller, etc. This leads to higher costs of the apparatuses. In addition, the special arrangement adds complexity to the apparatuses and hampers efforts to reduce the size of the apparatuses.